In vivo and in situ NMR studies are carried out on systems ranging from cell suspensions to perfused organs, to intact, anesthetized experimental animals in order to determine the mechanisms by which environmental chemicals and other types of stress irreversibly injure cells. Physiological, biochemical, and magnetic resonance measurements are carried out in parallel when possible, both to validate the techniques used, and more importantly, to correlate various metabolic changes in order to determine which factors may play a causative role. It has recently been suggested that perturbations in the level of cytosolic calcium ions may be of central importance in the production of irreversible cell injury arising from a variety of agents as a consequence of the activation of various catabolic, calcium dependent enzymes such as calpain and phospholipases. The level of cytosolic calcium ions in various preparations, particularly perfused rat hearts, has been determined based on the use of fluorine-19 NMR measurements of hearts loaded with fluorinated calcium chelator. Using this approach, the diastolic calcium level was determined to be 150 mM. Cytosolic calcium rose to approximately 2.5 uM after 15 minutes of ischemia, a time period shorter than required to irreversibly injure the heart, as determined by the leakage of cellular enzymes. Manipulations such as the introduction of calcium channel blockers, cardioplegic arrest, and low temperature, which delay the onset of irreversible injury, all correspondingly delay the rise of cytosolic calcium.